Quadrant detectors have been used for many years as direction finding devices in tracking systems and the like. The name "quadrant", of course, implies that the four adjacent quadrants of a circular detector subtending the entire field of view each comprise a separate radiation detector. If radiation from a point source is falling on the array of four quadrant detectors from a given angle whether on or off the viewing axis orthogonal to the plane of the detectors, the basic algebra by which the signals from the four quadrant detectors can be processed to determine the angular direction of the point source with respect to the plane of the quadrant detectors and its central viewing axis is well known in the art.
A simple quadrant detector may be provided with an objective lens. It is then found that the system has a limited field of view which depends in part on the detector size, as is shown in greater detail in FIG. 1. In order to increase the field of view, one must increase the detector size. In the usual system, however, one quickly finds that this method of increasing the field of view soon becomes impractical. The reason is that as one increases the detector size, the detector capacitance also increases and the detected signals, which are usually pulses, tend to disappear. Some other means must be found to increase the field of view. Commonly, the solution employed is to gimbal the narrow field of view detector with an appropriate gimbal scan search mode to acquire the desired target. After lock-on is achieved, the narrow field of view quadrant detector is used to track the target. This is obviously a complex, expensive solution. It would be far preferable to find a quadrant detector configuration which increases the field of view for a given detector size.
Holographic field lens reflectors have been used for this purpose. These are better than conventional quadrant detectors, but still do not achieve the field of view and detection efficiency attainable by the present invention.
Simple hemispherical single immersion lenses at a detector plane have also been used in the past to partially increase the field of view of a detector. Reference is made to a book entitled, "Modern Optical Engineering" by Warren J. Smith, published by McGraw Hill Book Company in 1966. At page 230, paragraph 9.7 describes radiometers and detector optics and on page 233 the typical arrangement of a single hemispherical immersion lens in a detector system between the detector and the objective lens is illustrated and analyzed.
It is an object of the present invention to provide a quadrant detector configuration which more nearly reaches the theoretical maximum field of view for a given detector size and which gives even greater field of view improvement than does the single hemispherical immersion lens system or the holographic field lens reflector.